Method for determining the position of a mobile station

ABSTRACT

The present invention relates to a base station of a cellular radio system, which base station comprises antenna equipment for receiving signals from a certain mobile station simultaneously by at least two antenna beams directed in different directions, and measuring equipment for measuring the signal levels of the signals received by the respective antenna beans. For determining the position of the mobile station with greater accuracy the base station is provided with calculating means which are responsive to the measuring equipment to determine the direction from the base station to the mobile station by calculating the relations between the signal levels of the signals for the respective beams.

FIELD OF THE INVENTION

The invention relates to a method for determining the position of amobile station located in the coverage area of a base station in a radiosystem, in which method the base station comprises equipment forreceiving signals from the same mobile station simultaneously by atleast two antenna beams directed in different directions, and in whichmethod: the signal levels of the signals received by the differentantenna beams are measured, the signal levels of the signals receivedfrom the same mobile station by the different antenna beams arecompared, and the direction to the mobile station in relation to thebase station is determined on the basis of the relations between thesignal levels measured for the different antenna beams. The inventionfurther relates to a base station of a radio system, which base stationcomprises antenna equipment for receiving signals from a certain mobilestation simultaneously by at least two antenna beams directed indifferent directions, measuring equipment for measuring the signallevels of the signals received by the different antenna beams,calculation means which are responsive to the measuring equipment fordetermining the direction from the base station to the mobile station onthe basis of the relations of the signal levels measured for thedifferent antenna beams, and equipment for defining a timing advance forthe mobile station, which is in radio connection with the base station,to compensate for a time lag caused by the distance between the mobilestation and the base station.

By the notion antenna beams turned in different directions is here meantthat the radio cell covered by the base station is divided into adjacentsectors from which signals related to the same logical channel (the samefrequency channel and time slot) are received, and that a directionalantenna or the equivalent is directed to each sector, by which antennasignals can be received from the sector in question. The antennas of thebase station are, however, preferably directed so that they overlap atleast partly in the border zones between them.

The invention relates to determining of the position of a mobile stationin a cellular radio system, e.g. the GSM system (Groupe Special Mobile).Prior solutions are known where the position of the mobile station has,for example, been determined by checking from the home location register(HLR) of the mobile switching centre of the GSM-system in which radiocell the mobile station is located at a certain moment. The mostsignificant problem with this known solution is its inaccuracy. Since itcan only be ascertained in which radio cell the mobile station islocated in the known solutions mentioned before, the accuracy ofdetermining the position, of course, directly depends on the size of theradio cell in question. The size of the radio cells again totallydepends on the properties of the radio system, but in the GSM system,for example, the inaccuracy of determining the position according to theradio cell can typically be several kilometers.

However, in practice there has been a need for greater accuracy indetermining the position of a mobile station. For example in connectionwith handover operations it would be necessary to determine the exactgeographical position of the mobile station. At present handover is, forinstance in the GSM system, based on the signal level and quality of thesignals received, and by no means on the position of the mobile station.Thus temporary radio disturbances can lead to an unnecessary handoveroperation, i.e. the mobile station is transferred from one cell toanother when the signal level or quality falls below a predeterminedlevel, after which the handover operation is repeated but in the reversedirection, i.e. the mobile station is returned to the original cell whenthe disturbance is over

Another situation where it would be necessary to determine the exactposition of the mobile station is when a stolen mobile station or forexample a SIM Card (Subscriber Identity Module) is to be located. Inknown solutions, where the inaccuracy in locating the mobile station isseveral kilometers, it is practically impossible to determine theposition of a stolen mobile station.

SUMMARY OF THE INVENTION

The object of the present invention is to solve the above mentionedproblems and to achieve a more precise method for determining theposition of a mobile station. This aim is achieved by the method of theinvention, which is characterized in calculating the distance from themobile station to the base station on the basis of a timing advancegiven to the mobile station by the base station and the propagationspeed of the radio signals.

The invention also relates to a base station by which the method of theinvention can be carried out. A base station according to the inventionis characterized in that the calculation means comprise equipment forcalculating the distance between the base station and the mobile stationon the basis of the timing advance defined for the mobile station andthe propagation speed of the radio signals.

The invention is based on the realization that the position of themobile station can be determined with significantly greater accuracythan in known solutions when its signals are received by at least twoantenna beams directed in different directions and when the audibilityof the signals received by the respective beams from the mobile stationis compared. In other words, the audibility of the signals transmittedby the mobile station is normally best for the beam that is directedstraight towards the mobile station. Thus it can be determined withinwhich beam the mobile station is located. When it is further known inwhich direction the beam in question is turned the direction to themobile station can easily be determined. How near the centre orrespectively the edges of the beam the mobile station is located can bedetermined by comparing the signal levels of the signals received by thebeam in question to the signal levels of the signals received by “theprincipal beam”. Thus the direction from the base station to the mobilestation can be determined from the relation between the signal levels ofthe signals received by the respective beams. In addition to this thedistance from the mobile station to the base station can, according tothe invention, be calculated on the basis of a timing advance given tothe mobile station by the base station and the propagation speed of theradio signals. In the GSM system, for example, there is already in use aso called timing advance TA given by the base station to the mobilestation to inform it of how much in advance it should transmit itssignals so that the signals will arrive at the right moment and in theproper time slot to the base station regardless of the distance betweenthe mobile station and the base station. Thus, the distance between thebase station and the mobile station can be determined on the basis ofthe timing advance when the propagation speed of the signals is known.

The most significant advantage with the solution of the invention isthus that the position of the mobile station, that is both its directionand distance from the base station, can be determined with significantlygreater accuracy than previously, which among other things makes itpossible to make handover decisions on the basis of the location of themobile station, whereby unnecessary handover operations can be avoided,and for example to locate a stolen mobile station with greater accuracythan before.

So that temporary disturbances would not significantly disturb thedetermining of the position of the mobile station, in a preferredembodiment of the invention the mean value of the signal levels of thesignals received by the respective antennas from a certain mobilestation is calculated for a certain time period, whereby the position ofthe mobile station is determined on the basis of the mean values of thecalculated relations.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the method and base station of theinvention are revealed in the attached dependent claims 2-4 and 6-8.

In the following the invention will be described in more detail in a fewpreferred embodiments by mean of the attached drawings, in which

FIG. 1 shows a flowchart of a first preferred embodiment of the methodof the invention,

FIG. 2 illustrates a first preferred embodiment of a base station of theinvention,

FIG. 3 shows an enlargement of the receiving beams of the base stationin FIG. 2, and

FIG. 4 shows a block diagram of the base station in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a block diagram of a first preferred embodiment of themethod of the invention. The block diagram in FIG. 1 can, for example,be applied in a base station of the GSM system to determine the positionof a mobile station.

In block A signals are received from a mobile station MS by severalantenna beams directed in different directions. The antenna beams usedare preferably relatively narrow beams that are directed so that they atleast partly overlap (compare with FIG. 2).

In block B the received signal strength indication RSSI of the receivedsignal is measured for the signals received by the respective beams.

In block C a mean value is calculated for the RSSI values measuredwithin a certain time span for each beam. By calculating the mean valuefor the RSSI values it can be avoided that temporary disturbancesinfluence the locating of the mobile station. For example in the GSMsystem the time span in question can be chosen so that the mean value iscalculated for a few bursts received from the mobile station.

In block D a first beam is chosen which has the highest RSSI mean value(=RSSI1). In addition to this at least one of the adjacent beams ischosen as a second beam, whereby preferably the beam with the higherRSSI mean value (=RSSI2) is chosen.

In block E the RSSI values for the different beams are compared bycalculating the ratio of the RSSI mean values RSSI1/RSSI2 for the chosenbeams.

In block F it is checked if the RSSI ratio for the beams is greater thanthe predetermined reference value K. The reference value is chosen sothat it is essentially greater than 1. If the RSSI ratio exceeds thereference value that denotes that the audibility of the mobile stationis much better by the first chosen beam than by the second chosen beam,which means that the mobile station is located in the direction of thecentre of the first beam. Hereby a transfer is made to block G, wherethe direction of the first chosen beam is specified, which is thedirection where the mobile station is located.

In block H it is checked if the RSSI ratio is nearly 1. If that is thecase it denotes that the audibility of the mobile station is almostequally good via both the chosen beams. This again means that the mobilestation is located in the border zone between the two beams. Hereby atransfer is made to block I, where the direction is specified where theborder zone between the beams (and the mobile station) is located.

Provided that the RSSI ratio of the beams is not greater than thereference value K, neither the RSSI ratio nearly 1, a transfer is madeto block J. Thus it is ascertained that the audibility of the mobilestation is somewhat better via the first chosen beam than via the secondchosen beam, which means that the mobile station is located between thecentre of the first chosen beam and the border zone between the chosenbeams. If it is necessary to determine the direction to the mobilestation more accurately than that, the direction in question can bedetermined by the RSSI ratio of the beams. That requires measurementsmade in advance so that a precise picture can be had of how the RSSIratio between the beams changes when the mobile station moves from theborder zone between the beams to the centre of the first beam.

By following the flowchart in FIG. 1 only the direction from the basestation to the mobile station can be determined. In addition to this itcan be necessary also to determine the distance from the base station tothe mobile station. According to the invention the distance in questioncan be calculated on the basis of the timing advance given to the mobilestation by the base station, that isdistance=timing advance*propagation speed of the radio signals

How accurately the position of the mobile station can be determined inrelation to the base station of course depends on the width of theantenna beams used and how accurately the base station calculates thetiming advance for the mobile station. For example in the GSM system theposition of the mobile station can be determined as described above withan accuracy of typically about 0.5×0.5 km, when the width of the antennabeams is about 30°.

FIG. 2 illustrates a first embodiment of a base station of theinvention. The base station BTS1 in FIG. 2 can, for example, be a basestation in the GSM system, which base station comprises equipment forreceiving signals transmitted by the mobile station MS from the radiocell in the figure simultaneously by four receiving beams 1-4. In FIG. 2the boundaries of the radio cell have been illustrated by a dash line R.

The mobile station MS of FIG. 2 is located in the border zone betweenbeams 1 and 2, whereby its audibility is almost as good by beam 1 as bybeam 2. That is the RSSI ratio for beams 1 and 2 calculated by the basestation BTS1 is nearly 1.

FIG. 3 shows an enlargement of the receiving beams 1 and 2 of the basestation in FIG. 3. It is assumed that the base station BTS1 has receivedthe signals with greater signal strength via beam 1. In that case themobile station that has transmitted the signals in question is locatedin the area covered by beam 1. If the mobile station is located in thecentre of beam 1, that is in the striped area A1 in FIG. 3, the basestation will recognize that as the calculated RSSI values for beam 1 areconsiderably greater than those for beam 2. That is the RSSI ratio isessentially greater than 1.

If the mobile station is located in the border zone A2 between the beamsthe base station will recognize that as the RSSI values calculated forbeams 1 and 2 are essentially as great, that is the RSSI ratio is nearly1.

If the mobile station is located between the centre A1 of beam 1 and theborder area between beams 1 and 2, that is in the area A3, the basestation will recognize that as the RSSI values measured for beam 1 aresomewhat greater than the RSSI values measured for beam 2.

FIG. 4 shows a block diagram of the base station in FIG. 2. The signalsrelated to the same logical channel that are received by the basestation BTS1 by the antenna beams 1-4 are fed through band-pass filters7 and amplifiers 6 to the RSSI receiver 8 of the base station. In FIG. 4the RSSI receiver 8 is shown in connection with the base station, but tofacilitate cabling the RSSI receiver can also be arranged in connectionwith an antenna element in the antenna mast of the base station.

In the example in FIG. 4 there are 4 inputs and one output. The RSSIreceiver chooses one of the signals fed into its inputs for furthertransmission via its output to the receiver RX of the base station. Inthe base station in FIG. 4 the RSSI receiver 8 chooses a signal forfurther transmission by measuring the signal level RSSI for the signalsreceived by each antenna beam 1-4 and by choosing the beam for which thegreatest RSSI value has been measured. Alternately the RSSI receiver canchoose the beam also in some other way, by also including a valuerepresenting signal quality like the bit error ratio BER in addition tothe RSSI value. A solution like that will, however, complicate theconstruction of the RSSI receiver.

If such a receiver is used that can manage diversity reception, the RSSIreceiver can have two outputs, whereupon the RSSI receiver chooses thetwo best antenna beams for use and transmits the signals received bythese further to the actual receiver of the base station. With anarrangement like this an improvement of about 3 dB can be achieved inthe reception of signals from a mobile station in the border zonebetween the sectors.

According to the invention the RSSI receiver 8 feeds the RSSI valuesmeasured for the respective beams to the calculator 9. The calculator 9can for example be a processor and a computer program added to the basestation for locating the mobile station.

The calculator 9 calculates the mean value of the received signals foreach antenna beam on the basis of the measuring results. Since the basestation in FIG. 4 is a base station of the GSM system, where thefrequency channels have been divided into time slots according to theTDMA principle, the calculator 9 first calculates a mean value for eachantenna beam separately for each logical channel. After this thecalculator specifies the direction from the base station to the mobilestation as shown in the flowchart in FIG. 1.

A signal denoting the timing advance TA given to the mobile station bythe base station is according to the invention also fed to thecalculator 9. On the basis of this signal the calculator calculates thedistance to the mobile station as described in connection with theflowchart in FIG. 1.

The signal POS fed from the output of the calculator thus denotes theposition of the mobile station in relation to the base station(direction+distance). For instance handover operations can be timedbetter on the basis of this information, since they are then based oninformation about the position of the mobile station. Additionally theinformation about the position of the mobile station can be transmittedfurther via the base station controller and the mobile switching centreto the network management centre, whereupon the operator can determinethe position of the mobile station with even greater accuracy from thenetwork management centre.

It is to be understood that the above description and the relateddrawings are only intended to illustrate the present invention. Thus theinvention can also be applied in other cellular radio systems than theGSM system. To those skilled in the art various other modifications andvariations of the invention will be apparent within the scope and spiritof the present invention disclosed in the attached claims.

1. A method for determining the position of a mobile station located ina coverage area of a base station in a radio system and for usinglocation information, in which method the base station comprisesequipment for receiving signals from the same mobile stationsimultaneously by at least two antenna beams directed in differentdirections, the method comprising: measuring signal levels of signalsreceived from a same mobile station by different antenna beams of saidbase station, comparing the signal levels of the signals received fromthe same mobile station by the different antenna beams, determining adirection to the mobile station in relation to the bas station on thebasis of a relations between the signal levels measured for thedifferent antenna beams, and calculating a distance from the mobilestation to the base station on the basis of a timing advance, given tothe mobile station by the base station and propagation speed of theradio signals, wherein said distance and said direction is used formaking a decision whether or not said mobile station should betransferred to another base station by a handover.
 2. A method accordingto claim 1, wherein calculating a mean value for the measuring resultsduring a determined time period and determining the direction to themobile station on the basis of the relations between the calculated meanvalues.
 3. A method according to claim 1, wherein choosing a beam bywhich signals with the strongest signal level have been received and atleast one of the adjacent beams (D), comparing the measured signallevels for the antenna beams, and determining the direction to themobile station on the basis of the relation between the signal levelsfor the chosen antenna beams.
 4. A method according to claim 1, whereindetermining that the mobile station is located in the centre of thefirst chosen beam, if signal level (RSSI1) of the signals received bythe beam is essentially higher than a signal level (RSSI2) of thesignals received by the other chosen antenna beam, in a border areabetween the antenna beams, if the signal level (RSSI1, RSSI2) of thesignals received by the chosen antenna beams are substantially the same,and between the centre of the first chosen antenna beam and the borderzone of the beams, if the signal level (RSSI1) of the signals receivedby the first antenna beam is somewhat higher than the signal level(RSSI2) of the signals received by the other antenna beam.
 5. Basestation (BTS1) of a radio system, which base station comprises: antennaequipment for receiving signals from a certain mobile stationsimultaneously by at least two antenna beams directed in differentdirections, measuring equipment for measuring the signal levels of thesignal received by the different antenna beams, equipment for defining atiming advance for the mobile station which is in radio connection withthe base station to compensate for a time lag caused by the distancebetween the mobile station and the base station, and calculation meanswhich are responsive to the measuring equipment for determining thedirection from the base station to the mobile station on the basis ofthe relations of the signal levels measured for the different antennabeams, and which calculation means comprise equipment for calculatingthe distance between the base station and the mobile station on thebasis of the timing advance defined for the mobile station and thepropagation speed of the radio signals, wherein said calculation meansare adapted to transmit said direction and said distance further in thesystem in order to be used for making a decision whether or not saidmobile station should be transferred to another base station by ahandover.
 6. Base station according to claim 5, wherein that thecalculation means are arranged for calculating for each beam the meanvalue of the signal levels of the signals received from the mobilestation by the respective antenna beams, whereby the calculation meansare arranged to determine the direction from the base station to themobile station on the basis of relations between the calculated meanvalues.
 7. Base station according to claim 5, in wherein that thecalculation means include means for choosing the antenna beam (1) withthe strongest signal level and at least one adjacent beam (2), whereinthe calculating means are arranged for determining the direction fromthe base station to the mobile station on the basis of the relations ofthe signal levels (RSSI1, RSSI2) of the signals received via the chosenantenna beams (1, 2).
 8. Base station according to claim 5, wherein saidbase station is a base station of a cellular radio system divided intological traffic channels in accordance with a TDMA principle.